1. Field of the Invention
This invention relates to a system for discovering defects in liquids, and more particularly to a system for detecting bubbles in a liquid being applied to a surface of a semiconductor wafer.
2. Background of the Technical Art
Liquids are used in a broad range of manufacturing processes. In one class of processes, a liquid is applied to a workpiece as a coating to form a temporary or permanent layer used in making the product. Such products include optical lenses and filters, magnetic disk surfaces for computers, compact disk surfaces, abrasives and adhesive films for surfaces. Of particular interest in this discussion are liquids used to manufacture semiconductor wafers for the electronics industry, where the ultimate products are integrated circuits (ICs) formed from these wafers. Unless the liquids used in fabricating semiconductor wafers satisfy stringent chemical, physical and electrical specifications, production yields of working IC's will be less than desired.
A common fabrication technique is the spin-on coating (SOC) of selected liquids. In a SOC process, a liquid, such as photoresist or a dielectric, is applied to a spinning wafer to produce a thin coating on the wafer. One type of spin-on coating process, called spin-on glass (SOG) coating process, forms a dielectric glass layer, such as a layer of silicon dioxide, on a wafer surface. In another type of SOC process, photoresist material is applied as a coating to an exposed surface.
Problems in SOC processes arise if the liquid supplied to a workpiece contains gas bubbles. Bubbles can reduce the dose of liquid applied to a wafer surface, and can cause irregularities in the resultant film.
Using present technology, the presence of gas bubbles (referred to simply as "bubbles" here) in a deposited liquid is not detected until after processed wafers are completed and when low yields become apparent. This is because present-day delivery systems are sealed to prevent contamination. For example, an SOC liquid is typically delivered to a production facility in a sealed bottle. This bottle is tapped for pumping the liquid by a liquid-handling system. The liquid-handling system is also sealed to minimize exposure of the process liquid to contaminants.
Bubbles can be present in the liquid when delivered, and can also be introduced into the liquid during transport from the bottle to the application point. Also, in present SOC systems, a small air gap separates the end of the liquid pipeline from the workpiece. The liquid is therefore exposed to contaminants present in air when crossing that gap, and may pick up particles or gas bubbles.
What is needed is a system for detecting and preferably removing bubbles in a liquid reagent, thereby improving process yields during the production of integrated circuits.